This invention relates to the control of power levels of transmitted signals in telecommunication systems, in particular spread spectrum multiple access systems.
Good transmit power control methods can be important to communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in communication systems that use code division multiple access (CDMA). This is important for the uplink, i.e., for transmissions from a remote terminal to the network, e.g., a base station. Uplinks are also sometimes called reverse links.
In a typical CDMA system, an information data stream to be transmitted is impressed upon a much-higher-bit-rate data stream produced by a pseudorandom code generator. The information signal and the pseudorandom signal are typically combined by multiplication in a process sometimes called coding or spreading the information signal. Each information signal is allocated a unique spreading code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and decoded.
The need for transmit power control in the uplink is recognized in current CDMA cellular systems, as may be seen from "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA Interim Standard TIA/EIA/IS-95 (July 1993) and its revision TIA/EIA Interim Standard TIA/EIA/IS-95-A (May 1995). Such standards that determine the features of U.S. cellular communication systems are promulgated by the Telecommunications Industry Association and the Electronic Industries Association located in Arlington, Va.
Uplink power control according to the IS-95-A standard is provided by a closed-loop method, in which a base station measures the strength of a signal received from a remote station and then transmits one power control bit to the remote station every 1.25 milliseconds. Based on the power control bit, the remote station increases or decreases its transmit (uplink) power by a predetermined amount. According to Sections 6.1.2.3.2 and 7.1.3.1.7 of the standard, a "zero" power control bit causes the remote station to increase its transmit power level by 1 dB and a "one" power control bit causes the remote station to decrease its transmit power level by 1 dB. The IS-95-A standard also addresses uplink power control in other situations, such as when a remote station accesses system (before the closed-loop power control method is active), but these are not pertinent to this application.
The need for transmit power control for the downlink, i.e., for transmissions from the network to a remote station, has been deemed less important in current cellular and other CDMA communication systems. Downlinks are also sometimes called forward links. This may have been due in part to the fact that interference from other transmitters is a smaller problem for the downlink than it is for the uplink because, from a remote terminal's point of view, the interference fades coherently with the downlink signal intended for it. The need for downlink transmit power control may also have been misjudged because signals from a base station are mutually orthogonal in a communication system that complies with the IS-95-A standard, and hence a large part of the mutual interference at a remote terminal is orthogonal to the downlink signal intended for it.
Moreover, the IS-95-A standard specifies a communication system that is intended to handle only speech, resulting in symmetrical load on the uplink and downlink. Since it has usually been assumed that mutual interference in the uplink, not the downlink, limits system capacity, downlink transmit power control has been deemed less important. In future communication systems, services may not be symmetrical in the uplink and downlink, and thus it is important to optimize both links independently of each other.
A trivial form of downlink power control would be provided by a communication system in which a remote terminal measures its received downlink power level and simply reports the measurement to a base station, which might adjust its transmit power in predetermined circumstances. Such a communication system is among those described in International Patent Publication No. WO 95/12297 by Gilhousen et al., which also describes a communication system in which downlink transmit power level is reduced by a predetermined amount based on frame error rate measurements, received uplink power levels, or received downlink power levels.
Downlink power control in a system compliant with the IS-95-A standard is based on frame error rate (FER) measurements by the remote station, which sends FER reports to the system. Sections 6.6.4.1.1 and 7.6.4.1.1 of the IS-95-A standard note that such FER reports can be sent when a threshold has been crossed and/or periodically. (Typically, an FER report would be sent every 1-5 seconds.) One problem with this method is that it can take a long time (several seconds) to accumulate the appropriate FER statistics. As a result, it is impossible to track Rayleigh fading and shadow fading. The method has proved to be so slow that it is usually attributed hardly any gain compared to not using downlink power control.
Some newer personal communications systems (PCS) also use CDMA. The features of U.S. PCS systems are specified in "Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems", ANSI J-STD-008 (August 1995), which is similar to the cellular IS-95 standard in many respects. For operation with rate set 2, however, the J-STD-008 standard requires the remote station to report downlink frame errors whenever they occur. This puts the network in complete control of frame errors, but it still takes a long time to accumulate the appropriate statistics, yielding only slight improvement over the IS-95-A standard's method.
In other communication system concepts like CODIT, which is described in "Final Report on Radio Subsystem Functionality", R2020/CSE/LC/DS/P/047/al, UMTS Code Division Testbed (CODIT), CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. ed. (August 1995), the signal quality is determined by estimating the raw bit error rate (BER) instead of the FER. Hence, good statistics can be obtained faster, and a remote station sends BER reports to the network more often (typically, 1-10 times per second). System performance is considerably improved in comparison to a system using downlink transmit power control according to the IS-95-A standard, but the CODIT method is still too slow to handle Rayleigh fading.
One might use the uplink transmit power control method described in the IS-95-A standard for transmit power control in the downlink. This is described in European Patent Publication No. 0 680 160 by Dohi et al. The remote terminal would then measure the downlink signal to interference ratio (SIR) and transmit an appropriate power control command on the uplink. In accordance with the IS-95-A standard, each power control command would be a single bit that was uncoded in order to minimize signaling overhead. Nevertheless, the communication system to which European No. 0 680 160 is directed has some significant differences from the system specified by the IS-95-A standard. For example, the European system has a frame length that is half that of IS-95-A, a bit rate of several hundred kilobits per second, a wider channel bandwidth of 5 MHz, and a CDMA chip rate of four million chips per second.
Such a communication system would track Rayleigh fading fairly well, and might work well when the remote terminal is not in a soft-handoff mode, i.e., when the remote terminal is not communicating with two or more base stations simultaneously. Soft handoff is described in U.S. Pat. Nos. 5,109,528 to Uddenfeldt and 5,327,577 to Uddenfeldt, both of which are expressly incorporated here by reference. When the remote terminal is not in soft-handoff mode, the error rate of the uncoded power control commands would typically be about one percent, which would not cause any great problems.
Nevertheless, the error rate of the uncoded downlink power control commands can be expected to increase significantly when the remote terminal is in soft-handoff mode. In addition, the errors in the commands received in different base stations involved in the soft handoff will be almost independent. Since 1600 power control commands per second would be sent to two base stations involved in a soft handoff according to the system described by Dohi et al., the commanded transmit power levels of the base stations can be expected to drift with respect to each other to levels that may be suboptimal from a system-capacity point of view. The lost capacity occurs because at least one of the base stations in communication with a remote station in soft-handoff mode will transmit at a power level that is too high.